1. Field
The present disclosure relates generally to fluid filtration and more particularly to methods for manufacturing membrane filters for the removal of fine particulates from fluid streams.
2. Technical Background
Ceramic wall flow filters are presently employed for the removal of particulates from fluid exhaust streams such as power plant stack gases and combustion engine exhausts. Examples include the ceramic soot filters used to remove unburned carbonaceous particulates from diesel engine exhaust gases. Present diesel particulate filters, or DPFs, consist of honeycomb structures formed by arrays of parallel channels bounded and separated by porous channel walls or webs, with a portion of the channels being blocked or plugged at the filter inlet and the remaining channels being plugged at the filter outlet. Exhaust gas to be filtered enters the unplugged inlet channels and passes through the channel walls to exit the filter via the unplugged outlet channels, with the particulates being trapped on or within the inlet channel walls as the gas traverses the filter.
Standard gasoline engines do not require exhaust filtration, but gasoline direct injection (GDI) engines, which are more fuel-efficient than standard gasoline engines, do emit some soot particles of fine particle size. For this reason, and due to tightening environmental regulations governing exhaust emissions from motor vehicles, stricter limits on particulate emissions from gasoline engines are to be expected.
Current diesel particulate filters have porosities that efficiently trap the relatively large soot particulates produced by diesel engines. However, they are less efficient than would be desirable for the collection of particulates of very fine particle size, and in addition impose fuel consumption penalties due to the higher exhaust backpressures generated by the filters. Gasoline engines will likely require filters of higher trapping efficiency than offered by conventional diesel particulate filters.
There is a well-known trade-off between filtration efficiency and backpressure. From the viewpoint of high filtration efficiency, a particle filter of small pore size and large filtration wall thickness is preferred, while from the viewpoint of low backpressure, large pore size and small wall or web thickness are more desirable. One filter design that has been proposed to increase filtration efficiency without a large pressure drop penalty is the so-called membrane filter, wherein a relatively thin membrane of small pore size is applied to a supporting filter wall of higher thickness but larger pore size.
For honeycomb membrane filters intended for high temperature use, the membranes are typically formed of thin porous layers of a refractory ceramic or glass material, with or without a catalytically active metal component, and are generally applied via slurry coating to either the inlet channels or outlet channels of the structure. The simultaneous coating of all channel walls would be less expensive from a processing standpoint, but more expensive from the standpoint of materials costs, and both unnecessary and disadvantageous from the standpoints of filtration efficiency and filter pressure drop.
Present processes being considered for membrane filter fabrication generally comprise feeding or drawing a membrane coating slurry into the inlet or outlet channels of a pre-plugged honeycomb body, drying the resulting coating, and firing the coated honeycomb to a temperature sufficiently high to consolidate and bond coating constituents into an adherent membrane layer. Problems with these processes are several, including high processing costs and uneven membrane thickness. A further problem is that the heating required to consolidate and bond the membrane coating to the channel walls can damage plug integrity and/or loosen the bonds formed between the plugs and the honeycombs, creating filter durability and/or leakage problems.